Method for noise correction for a flat-panel detector

ABSTRACT

For noise correction in connection with a flat-panel x-ray detector, noise signals of a dark reference area are checked for deviations exceeding a specified threshold which, if any are present, will be taken into consideration separately for calculating the correction factor derived from the noise signal. Image artifacts due, for example, to high-contrast objects such as, for instance, cardiac pacemakers or metallic implants, in the x-ray image will be avoided through this measure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 11,092,795 filedMar. 29, 2005 now U.S. Pat. No. 7,965,333 and claims priority to theGerman application No. 10 2004 016 587.4 filed Mar. 31, 2004. All of theapplications are incorporated by reference herein in its entirety.

FIELD OF INVENTION

The invention relates to a method for noise correction in theacquisition of, in particular, x-rays by means of a flat-panel detector.

BACKGROUND OF INVENTION

A flat-panel detector of said type usually has an amorphous silicon(aSi) flat panel that is divided into individual pixel arrays and ofwhich at least a subarea is provided with a photo-active coating formingthe scintillator layer. Said photo-active coating converts impingingx-rays into light pulses which are then detected by a semiconductorsensor assigned to the respective pixel array and forwarded as anelectrical signal to a readout amplifier. Each of the pixel arraysarranged in lines and columns has a semiconductor sensor of said type.The individual pixel arrays form a matrix array.

A plurality of readout amplifiers are usually provided which in turnhave a plurality of channels, for example 120, for the respectivecolumns of the matrix array. One column is therefore read out via eachchannel, this process taking place line by line, which is to sayserially. The detected light intensity is aggregated in the individualsemiconductor sensors during the time that elapses between two readoutoperations.

SUMMARY OF INVENTION

Owing to the plurality of electronic components employed, in particularowing to the readout amplifiers, the actual light signals aresuperimposed with noise, in particular line noise. In order to reducesaid noise, the flat-panel detector is usually subdivided into an activearea, which is exposed to the x-rays, and at least one dark area,referred to also as a “Dark Reference Zone (DRZ)”, which in particulardoes not have a photo-active layer. The noise to be assigned to the darkarea is detected and used as a correction value for the signals detectedby the active area (line noise correction: LNC).

If a high-contrast object such as a cardiac pacemaker or metallicimplant is located in the path of the rays at the edge of the image,which is to say at the edge of the active area and in the vicinity ofthe dark area, there will be an area of very high light intensity in theimmediate vicinity of the dark area. Said light can as a result alsoradiate into the dark area so that significantly increased signal valueswill be registered by the corresponding readout amplifier. At the edgeof said object there will additionally be a high-contrast transitionfrom one line to the next line within a readout amplifier. This abruptchange in contrast will produce a kind of high-frequency structure thatwill in part be visible over the entire line. This effect is alsodescribed as matrix vertical high-pass behavior of the readoutamplifier.

Owing not least to radiation into the dark area, employed ultimately, ofcourse, for noise correction of the line noise, increased line noisewill be incorrectly assumed and noise correction will produceunsatisfactory results. Line structures that are undesirable anddisadvantageous for clinical image evaluation will as a result beproduced in the image.

An object of the invention is to enable improved noise correction.

Said object is achieved according to the invention by means of a methodfor noise correction in the acquisition of, in particular, x-rays bymeans of a solid-state flat-panel detector containing a matrix-typedetector panel having at least one dark area and one active area andfurther containing a plurality of readout amplifiers for reading outline by line the signals detected in the detector panel. Noise signalsfor a line noise that are to be assigned to the dark area are detectedand correction factors for the individual lines of the active areadetermined therefrom. The detected noise signals of the individual linesof the dark area are checked during this process for deviations thatexceed a specified threshold. Any such impermissible deviationsoccurring are taken into consideration separately for calculating thecorrection factor.

Through this measure, in particular those instances are thereforeregistered in which high-contrast objects are located in the area of theedge of the image, which is to say near the dark area, and which wouldcustomarily give rise to the undesirable line structures in the clinicalimage. It is as though by means of the method here proposed thoseparticular lines affected owing to a high-contrast object of said typeare identified into which, for example, light radiates. Recognizing theaffected lines in the first step is here the prerequisite for its beingpossible to take this into consideration in the second step forcalculating the correction factor in order to enable improved noisecorrection.

A mean value of the noise signal across the lines of the dark area ishere preferably used to determine the threshold characterizing adeviation as being an impermissible deviation. The mean value is inparticular here produced in the dark image, which is to say withoutimpinging x-rays, in order to ensure that only the electronic noiseactually contributes to the signal. For the purposes of determining, thelines are therefore read out line by line, a mean line value is formedfrom the noise signal curve of the line, and a total mean value thencalculated for the dark area from all the mean line values. Thethreshold is then determined by, for example, the mean value plus atolerance value. Noise signals within this tolerance range are thereforeidentified as being permissible so that the noise values or noisesignals can be used for determining the correction factor. Saidthreshold as a function of the mean value therefore forms a firstcriterion for the presence of an impermissible deviation.

According to a preferred embodiment an abrupt change in the noise signalbetween two successive lines is used additionally or alternatively as afurther, second criterion for the presence of an impermissibledeviation. Said second criterion is therefore very sensitive in the edgeareas of the high-contrast object in which the abrupt changes incontrast occur.

A dark area's noise signal to be assigned to a respective line iscomposed of a plurality of individual noise values to be assigned to thecolumns of the dark area. A check is here carried out to determinewhether individual noise values have an impermissible deviation.According to said preferred embodiment each pixel of an individual lineof a dark area is therefore considered separately. In order to avoiddistortions in determining the correction factor, noise valuesexhibiting an impermissible deviation of this type will preferably notbe used for determining the correction factor.

The entire line of the dark area will furthermore preferably not be usedfor determining the correction factor if more than a specifiedpermissible number of noise values have impermissible deviations. As thequality of the correction factor falls significantly with increasinglyfewer individual values, a deterioration in the correction factor isthereby counteracted.

In order nonetheless to facilitate a meaningful correction factor forthe affected line for the active area, the lines adjacent to the linehaving an impermissible deviation are used to determine the correctionfactors. The noise signal of, for example, the preceding line istherefore interpolated or the mean value is formed from two adjacentlines each not having impermissible deviations.

A predetermined correction factor in particular derived from the meanvalue of the noise signals of the lines of the dark area is usedalternatively for the lines exhibiting an impermissible deviation. Ameaningful correction factor can in this way be determined withcomparatively modest effort.

The above embodiments relate in particular to a detector panel havingonly one dark area. Two dark areas are in part also employed fordetermining the correction factor, said areas being customarily locatedopposite each other on the edge sides of the detector panel. For thisinstance, in a preferred embodiment a noise signal is registered for theline noise for each of the dark areas and each of the noise signals ischecked for impermissible deviations.

A mean line value is here to practical effect formed from the noisesignal for a line of a respective dark area. The difference between thetwo mean values is then used as a further, third criterion for thepresence of an impermissible deviation. This is therefore a relativecriterion which compares the two noise signals of the dark areas witheach other and draws conclusions therefrom. If, for example, there is ahigh-contrast object in an edge area, then the dark area adjacent tosaid object will have a significantly increased noise signal.

If there is an impermissible deviation for the noise signal for one ofthe dark areas, then only the noise signal of the further dark area orareas will to practical effect be used for determining the correctionfactor. When two dark areas are used it is therefore relatively easy toresort to the second, unaffected dark area for determining thecorrection factor.

The criteria and procedures proposed with regard to a detector panelhaving only one dark area can, of course, be applied equally to adetector panel having two dark areas. When an impermissible deviation isdetected, line noise correction will in particular be performed ineither case based on the detected line noise with the impermissibledeviation “eliminated”, and substitute noise correction will beperformed.

The effect of the “vertical high-pass behavior of the readout amplifier”in the area of the dark area can be detected and corrected using theline noise correction described here.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is explained in more detailbelow with reference to the drawing, in which, in schematic and highlysimplified representations:

FIG. 1 shows a flat-panel detector in the form of a block diagram, and

FIG. 2 shows a schematic signal curve across the bandwidth of a readoutamplifier for different situations.

DETAILED DESCRIPTION OF INVENTION

A flat-panel detector 2 according to FIG. 1 has a matrix-type detectorpanel 4 having a total of n lines and m columns. The detector panel 4has on its edge sides in each case a plurality of columns covering aradiation-insensitive dark area 6 and between said two dark areas 6 aradiation-sensitive, active area 8. Unlike in the area of the activearea 8, in the area of the dark areas 6 the detector panel 4 does nothave a photo-active or scintillator layer that generates light quantawhich are then detected by semiconductor detectors, not shown in moredetail here, when x-rays impinge. In operating modes in which a moreextensive subarea of the detector panel 4 is shaded by, for example,masking, the dark areas 6 can also extend to areas which, although inprinciple having a photo-active coating, are not irradiated in therespective operating mode.

The flat-panel detector 2 furthermore has a driving device 10 fordriving the detector panel 4 line by line. Each pixel array of thedetector panel 4 includes a semiconductor sensor for registering thelight intensity as a measure of the x-rays impinging on the respectivepixel array. A total of M sensors of this kind is therefore arranged inone line.

The detector panel 4 further has a plurality of readout amplifiers 12which in turn have a plurality of individual channels, for example 120,with each channel being assigned to precisely one of the total of mcolumns. The total of m columns of the detector panel 4 is thereforesubdivided into individual column blocks 14, as shown in FIG. 1, each ofwhich is assigned to a readout amplifier 12. As can further be seen fromFIG. 1, the dark areas 6 do not necessarily extend across a completecolumn block 14.

During operation the detector panel 4 is driven via the driving device10 line by line, which is to say that the detector panel 4 is driven insuch a way that one line is in each case read out serially after theother. Each readout amplifier 12 therefore receives line by line thesignal data of its columns of the respective column block 14 that areassigned to it. Owing to the evaluation electronics the actual imagesignals are overlaid by electronic noise emanating in particular fromthe readout amplifiers 12.

What is termed line noise correction (LNC) is performed in order to atleast largely eliminate said noise. A correction factor or correctionvalue is for this purpose determined for each line of the dark area 6.This is customarily the arithmetic mean of the individual noise valuesof the individual columns channels of the readout amplifier 12 of arespective line. Said correction value is then customarily deducted fromthe image signals recorded across the active area 8.

If a high-contrast object 16, for example a cardiac pacemaker ormetallic implant, is located near the image edge of the dark area 6, asis shown in FIG. 1 by the dark-hatched rectangle, it means there is anarea exhibiting very high radiation intensity. Owing to the abrupt andhigh-contrast transition at the edge areas of the high-contrast object16 from one line to the next line, on the one hand a type ofhigh-frequency structure is produced which is discernible in therespective line across the entire channel width of the left-hand readoutamplifier 12 a. This effect is described also as vertical high-passbehavior of the readout amplifier 12 a. It is here particularlydisadvantageous in terms of noise correction that said effect not onlyregisters the readout amplifier's columns (channels) across which thehigh-contrast object 16 is located. Rather it is the case that adjoiningcolumns/channels and customarily the entire readout amplifier 12 a arealso affected. The entire line of the detector panel 4 across allcolumns is even also in part affected. If this “defective” line of thedark area 6 is then used for determining a correction factor, that willgive rise to faulty noise correction. Overall, therefore, faulty linestructures can be produced in the clinical image if a high-contrastobject 16 of said type is present.

A first possibility for reducing said effect (vertical high-passbehavior of the readout amplifier 12 a) is for the dark area 6 to extendat least across the entire channel width of the readout amplifier 12 aand thus include a complete column block 14. That is because no abruptchange in contrast would in this case occur within the readout amplifier12 a as it does not, of course, have an “active area”. This isfacilitated by, for example, suitably collimating the path of the rayswith the aid of, for instance, masks. Or lead screening is provided inthe edge areas of the detector panel 4. Said measures requireconsiderable expenditure, however, and are therefore impractical.

To correct said effect the present invention provides for the noisesignals of the dark area 6 that are registered for noise correction tobe checked for impermissible deviations, which is to say for deviationsexceeding a specified threshold, and for this then to be taken intoconsideration for these lines in determining the correction factor orcorrection value. With this method, in a first step the possiblepresence of an undesirable effect of said type is therefore identifiedand in a second step the appropriate conclusions are then drawn fornoise correction. This method can be applied both to a detector panelhaving only one dark area and to detector panels 4 having a pluralityof, in particular two, opposite dark areas 6.

FIG. 2 shows an exemplary signal curve S across the entire bandwidth ofthe left-hand readout amplifier 12 a, specifically for two in particularsuccessive lines between which the abrupt change in contrast occurs. Thenormal line not yet exhibiting any impact of the high-contrast object 16is here shown as a dotted line, whereas the line affected by thehigh-contrast object 16 is shown as an unbroken line. The verticaldashed line here schematically subdivides the left-hand area of thereadout amplifier 12 a assigned to the dark area 6 from the right-handside of the readout amplifier 12 a assigned to the active area 8.

In the normal case (dotted line) and without radiation the signal curveS will vary somewhat around a mean value X. As an arithmetic mean of theindividual noise values of the channels of the readout amplifier 12 afor the dark area 6, said mean value X is at the same time a suitablecorrection factor for line noise correction. The horizontal dashed linesalso shown in the drawing here indicate an upper and lower tolerancerange within which a deviation from the mean value X is still consideredpermissible. In particular the upper dashed line defines a threshold orlimiting value g above which there is an impermissible deviation.

If the high-contrast object 16 is present, said object can be expressed,as shown by the unbroken line, to the effect that light also radiatesinto the actually non-sensitive dark area 6 so that significantlyincreased signal values are also produced here for the individualchannels/columns. If the detector panel 4 has only one dark area 6, acheck will to practical effect be performed to determine whether theindividual noise values for each column (individual pixel values) exceedthe permitted threshold 9. If they do, they will not be taken intoconsideration for noise correction. If the number of individual pixelvalues exceeding the permissible threshold 9 exceed a pre-specifiednumber so as to leave residual pixel values too low in number todetermine the correction factor, then the entire line will be left outof consideration for determining the correction factor. In that case aninterpolation will then be carried out from preceding or succeedinglines or a mean value will be formed from the correction factors ofadjacent lines or a predefined correction factor determined from, forexample, the mean value of all line-correction factors will be taken asthe basis. If there are two or more dark areas 6, the same procedure canbasically be carried out for each dark area. It is alternativelypossible to compare the two correction factors of the dark areas 6 witheach other and leave the higher correction value out of considerationfor line noise correction if a pre-specified difference between the twocorrection values is exceeded. Both correction values will alternativelybe left out of consideration and here, too, an interpolation carried outfrom adjacent lines or a mean value formed from adjacent lines, or apre-specified correction factor will be used.

A vertical high-pass behavior of the readout amplifier 12 a occasionedby a high-contrast object is in particular detected and effectivelycorrected in the areas of the dark areas 6 by means of theabove-described method so that image artifacts are largely suppressedand avoided. Comparable effects such as the radiation of light into thedark area are at the same time also detected and corrected by means ofsaid method.

The invention claimed is:
 1. A method of correcting noise included in animage acquired by a flat-panel-detector having a matrix-type detectorpanel including a plurality of lines, each for providing a plurality ofsignals, the matrix-type detector panel including a dark reference zoneand an active area, the method comprising: receiving the plurality ofsignals from a plurality of read out amplifiers; detecting, via thesignals, a plurality of dark noise contributions in the dark referencezone, each dark noise contribution related to one or more pixels in thedark reference zone on one or more of the lines of the matrix-typedetector panel; and providing a first procedure and a second procedurewhich is different from the first procedure; for each line of thematrix-type detector panel associated with the dark reference zone;comparing the detected values of dark noise with a threshold value,calculating correcting parameters based on the comparing, thecalculating is in accord to a first procedure when the comparisonindicates the detected values of dark noise contributions does notexceed the threshold value, the calculating is in accord to a secondprocedure when the comparison indicates the detected values of darknoise contributions exceeds the threshold value, and correcting theimage based on the flat-panel-detector using the correcting parameters,wherein the first calculation procedure provided for dark noisecontributions not exceeding the threshold value, and wherein the secondcalculation procedure provided for dark noise contributions exceedingthe threshold value.
 2. The method according to claim 1, wherein thethreshold value is based on a deviation from mean noise signal valuecalculated from the plurality of noise contributions.
 3. The methodaccording to claim 1, wherein the threshold value is determined using adetected signal step of a first noise level relative to a second noiselevel, the first and second noise levels related to a first respectivelya second line of the matrix-type panel, and the first and second linesarranged adjacent to each other.
 4. The method according to claim 1,wherein each dark noise contribution related to one line of thematrix-type detector panel includes a plurality of column dark noisecontributions related to the columns of the matrix-type detector panel,and a deviation of at least one column dark noise contribution from acolumn noise threshold value is determined.
 5. The method according toclaim 4, wherein column dark noise contributions deviating from thecolumn noise threshold value are omitted from the calculating of thecorrecting parameters.
 6. The method according to claim 4, whereincalculating the correcting parameters includes omitting all column darknoise contributions from calculating the correcting parameters if morethan a number of column dark noise contributions deviate from the columnnoise threshold value.
 7. The method according to claim 6, wherein thecalculating of the correcting parameters according to the secondprocedure includes the number of column dark noise contributionsdeviating from the column noise threshold value is based on correctingparameters calculated for adjacent lines.
 8. The method according toclaim 6, wherein calculating the correcting parameters for such linesincluding the number of column dark noise contributions deviating fromthe column noise threshold value is based on a correcting parameter. 9.The method according to claim 8, wherein the correcting parameterincludes a mean dark noise contribution value calculated from theplurality of dark noise contributions.
 10. The method according to claim1, wherein the matrix-type detector panel includes at least two darkreference zones, a plurality of dark noise contributions related to eachdark reference zone is determined, and calculating the correctingparameters is based on the plurality of dark noise contributions. 11.The method according to claim 10, wherein, for each dark reference zone,a mean line dark noise contribution value is calculated for each line ofthe dark reference zone, and the threshold value for each line isdetermined relative to the mean line noise value, and fore each line anarithmetic mean of all individual pixel dark noise contributions in theline is only used for calculating correcting parameters when none of theindividual dark noise contributions exceed the threshold value.
 12. Themethod according to claim 10, wherein the calculating of the correctingparameters omits all dark noise contributions related to a darkreference zone including dark noise contributions exceeding thethreshold value.
 13. The method according to claim 1, determining fromthe comparison for at least one line of the matrix-type detector panelassociated with the dark reference zone that the dark noise contributionexceeds the threshold value.
 14. The method according to claim 1,determining from the comparison for at least one line of the matrix-typedetector panel associated with the dark reference zone that the darknoise contribution does not exceed the threshold value.
 15. The methodaccording to claim 1, determining from the comparison for at least oneline of the matrix-type detector panel associated with the darkreference zone that the dark noise contribution exceeds the thresholdvalue, and determining from the comparison for at least one further lineof the matrix-type detector panel associated with the dark referencezone that the dark noise contribution does not exceed the thresholdvalue.